Retinoids are defined as a series of compounds which are natural derivatives or synthetic analogues of vitamin A. The role of vitamin A as an essential nutrient was recognised as early as 1913, since which time a great deal of research has been conducted on the product. Wolbach and Howe (J. Exp. Med. 42: 753,1925) first described the histopathological epithelial variations caused by vitamin A deficiency in 1925. This led to the identification of retinol and other natural analogues which began to be used, on a purely empirical basis, as chemopreventive agents of neoplastic transformation.
The role of retinoids in oncological chemoprevention was endorsed by the publication of various epidemiological studies which demonstrated that a regular vitamin A intake was significantly correlated with a lower incidence of tumours, especially lung cancer (Zeigler R. G. et al., Cancer Causes and Control 7: 157-177, 1996; Krishnan K. et al., Primary Care 25: 361-382, 1998).
Only some of the over 4000 retinoids tested to date have a sufficiently favourable therapeutic efficacy/toxicity ratio to allow their clinical use. The relatively recent discovery of nuclear receptors for retinoic acid (which belong to two distinct types, called RAR and RXR) has considerably improved knowledge of their action mechanisms.
Numerous clinical trials have been conducted with retinoids, many of them on skin diseases such as lichen planus and leucoplakia which, due to the high frequency of neoplastic transformation, are classed as pre-cancerous lesions (Hong W et al., N. Engl. J. Med 315: 1501-1505, 1986; Lippman S. M et al., N. Engl. J. Med 328: 15-20, 1993).
At present, the internationally recognised clinical use of the retinoids relates to the treatment of acute promyelocytic leukemia and skin diseases with a hyperproliferative component such as acne and psoriasis.
Fenretinide (4-hydroxyphenyl retinamide) is a semi-synthetic retinoid which was developed as a chemoprotective agent (Costa A. et al., Ann. N.Y. Acad. Sci. 768: 148-162,1995; Pienta K. J. et al., Am. J. Clin. Oncol. 20: 36-39, 1997).
Unlike other natural retinoids such as all-trans, 13-cis- and 9-cis-retinoic acid, fenretinide does not induce any systemic catabolism which could interfere with the long-term maintenance of pharmacologically useful plasma concentrations. This characteristic, combined with the low toxicity of the product and its ability to inhibit some phenomena associated with carcinogenesis, provides the rationale for the development of fenretinide as a chemoprotective agent in neoplastic disorders such breast, prostate and bladder cancer.
Other phase II trials, conducted on a limited number of subjects, have evaluated the effect of fenretinide on patients suffering from prostate cancer (Pienta K. J. et al., Am. J. Clin. Oncol. 20: 36-39, 1987), melanoma (Modiano M. R. et al., Invest. New Drugs 8: 317-319, 1990) and myelodysplastic syndromes (Garewal H. S. et al., Leukemia Res. 13: 339-343, 1989). However, the results of these studies were rather disappointing, whereas chemoprevention studies conducted on patients suffering from leucoplakia or lichen planus (dermatological lesions which often present neoplastic degeneration) have given promising results (Tradati N. et al., Canc. Letters 76: 109-111, 1994).
In these cases the patients were treated topically, with the result that the fenretinide concentrations reached in the lesion were probably similar to, if not higher than those which have proved active in vitro.
At present, topical formulations of retinoids are mainly presented in the form of creams with a fatty base or gels. DE 19946184 describes emulsions of retinoids characterised by a continuous aqueous phase, a mainly non-crystalline viscous oily phase, and a mixture of emulsifiers. Microemulsions of active ingredients which are poorly soluble in water and can be administered by the parenteral, topical or oral route are described in WO 99/56727, EP 211258 and EP 760237.
Topical formulations of retinoids in the form of microemulsions (macroscopically monophasic dispersed systems constituted by at least three components, such as an oily phase, an aqueous phase and a surfactant) are not available. The main chemico-physical properties of microemulsions which characterise their particular structure are transparency, isotropy and thermodynamic stability. As a result of these characteristics, microemulsions are of considerable interest to the pharmaceutical industry. In fact:
(a) the particular microstructure of microemulsions enables molecules with different chemico-physical characteristics to be solubilised;
(b) the transparency of the system makes it possible to check that the active ingredients are completely solubilised;
(c) thermodynamic stability entails major advantages, as the systems obtained are stable for long periods of time.
A further advantage of the topical use of microemulsions is the possibility of increasing the rate of penetration of the active ingredients through the stratum corneum.
Drug release is known to be much faster when gel microemulsions are used rather than conventional formulations (Martini M. et al., J. Pharm. Belg. 39, 348-354, 1984; Ziegerneyers J. et al., Acta Pharm. Technol. 26, 273-275, 1980; Ziegenmeyers J. et al., Deuxième Congrès International de Technologie Pharmaceutique 3, pp. 235-238, 1980).
Phospholipids have been used as emulsifying agents to stabilise microemulsions: phosphatidylcholines in an organic solvent (50-250 mM) form small inverse micelles which, on the addition of water, undergo one-dimensional growth, until the formation of a kind of three-dimensional network consisting of a tangle of long, flexible cylindrical structures (Luisi et al., Colloid Polym. Sci. 268, 356-374, 1990).
The presence of water causes a drastic increase in viscosity, leading to the formation of a gelified transparent system, the viscosity of which depends on the content of the aqueous phase. The water content, which is consequently a very important factor in the formation of these particular microemulsions, is expressed by the ratio between the water concentration and the molar lecithin concentration:[W]/[lec]=[molar water concentration]/[molar lecithin concentration]
The maximum value of [W]/[lec] for each microemulsion depends on the type of organic phase used and the lecithin concentration. The maximum viscosity of lecithin-based gel microemulsions is usually obtained after the addition of less than 10 molecules of water per molecule of lecithin, namely at values of [W]/[lec]<10.
Gel microemulsions based on soy phosphatidylcholine possess all the characteristics of transparency, thermodynamic stability and isotropy typical of microemulsions (Scartazzini R. et al., J. Phys. Chem. 92, 829-833, 1988; Luisi et al., Colloid Polym. Sci. 268, 356-374, 1990; Lawrence et al., Advanced Drug Delivery Reviews 45, 89-121, 2000).
Phosphatidylcholine is a natural surfactant, and is highly biocompatible (Dreher et al., Skin Pharmacology 9, 124-129, 1996).
Hyaluronic acid (HA) is a heteropolysaccharide composed of alternate residues of D-glucuronic acid and N-acetyl-D-glucosamine. It is a linear-chain polymer with a molecular weight ranging between 50,000 and 13×106 Da, depending on the source from which it is obtained and the preparation methods used. It is found in nature in pericellular gels, in the ground substance of the connective tissue of vertebrates (of which it is one of the main components), and in synovial (joint) fluid, vitreous humour and the umbilical cord.
HA plays an important role in biological organisms, as a mechanical support for the cells of many tissues such as skin, tendons, muscles and cartilage.
It is the main component of extracellular matrix, and also performs other functions such as tissue moisturising and cell lubrication, migration and differentiation.
In view of its properties of bio- and mucoadhesion and its tissue compatibility characteristics, hyaluronic acid and its salts, in particular sodium, potassium, magnesium and calcium salts, possibly suitably fractionated and/or derivatised, have been proposed as systems for the release of drugs and the preparation of surgical aids, implants, prostheses and the like.